1. Field of the Invention
This invention relates to improvements in a method and a system for searching for a reinforcing steel in a reinforced concrete, employed in the construction and building business and the authorities concerned, and more particularly to such a method and a system in which the reinforcing steel in the concrete is magnetized prior to a searching operation in order to greatly improve the sensitivity of detection.
2. Description of the Prior Art
Hitherto, three typical techniques set forth hereinafter have been proposed and known to search the reinforcing steel in the reinforced concrete.
(1) First conventional technique:
As shown in FIG. 15, a reinforcing steel searching instrument includes a sensor having a simple coil 61, in which an alternating current is passed through the coil 61 to generate lines of magnetic force which are formed through the opposite ends of the coil 61. This coil 61 is approached as the sensor to the surface of a concrete wall, in which a change is made in the lines of magnetic force if a reinforcing steel as a magnetic material exists in the concrete wall. This change is converted to a voltage or a current which is indicated in a visible manner in an indicator, thereby searching the existence of the reinforcing steel in the concrete.
(2) Second conventional technique:
As shown in FIG. 16, a reinforcing steel searching instrument includes a sensor having a core 65 which is made of a magnetic material and disposed inside the coil 61. Upon passing current through the coil 61, uniform lines of magnetic force is developed at the opposite ends of the core 65. The lines of magnetic force are radiated from the end of the core 65 and converged to the opposite end of the core. The sensor is encased to form a probe which is approached to and moved on the surface of a concrete structure 63 to search a location at which the lines of magnetic force are concentrated. If the reinforcing steel 64 is present there, the indication of a meter changes so that the location of the reinforcing steel is nearly determined. This conventional technique is slightly higher in sensitivity and precision of detection than the first conventional technique because of provision of the core 65 inside the coil 61 of the probe.
(3) Third conventional technique
This is a technique using a reinforcing steel searching instrument including a resonance circuit, as disclosed in Japanese Patent Provisional Publication No. 2-49187. In this instrument, a voltage at a predetermined frequency output from an oscillator is supplied to two circuit systems. One of the circuit systems includes a resonance circuit having a sensor coil and a variable capacitor, and an amplifier, whereas the other includes a phase converter connected in parallel with the resonance circuit, and an amplifier. The outputs from the two circuit systems are input to a differential amplifier to compare and amplify the outputs. The result of such comparison and amplification is input to a lock-in amplifier to pick up the output having the same frequency as the output frequency of the oscillator.
With this conventional arrangement, in case the sensor coil is used as a sensor or probe, the existence of the reinforcing steel is represented as an output differential between the two circuit systems. As a result, this technique is high in detection sensitivity and searchable depth of location of the objective reinforcing steel in the concrete, as compared with the first and second conventional techniques.
However, difficulties have been encountered in the above three conventional techniques as follows:
In the first conventional technique, since a sensor element consists of only the coil, the generated magnetic flux is not only weak but also spreaded laterally from the opposite ends of the coil, so that the magnetic flux is thin near the reinforcing steel to be searched, thereby lowering the sensitivity and precision of detection.
In the second conventional technique, the magnetic flux radiated from the core 65 extends generally in the axial direction of the core 65, so that the sensor unnecessarily senses a magnetic material located rearward of the sensor or probe. As a result, the resolution of the object to be searched is relatively low, and therefore this conventional technique is also low in sensitivity and precision of the detection.
In the third conventional technique, since the resonance circuit is used, the sensitivity and precision of detection is higher than the first and second conventional techniques. However, detection of the searching instrument of this conventional technique only depends upon a change in magnetic flux in the sensor coil which change is caused by the existence of the reinforcing steel in the concrete. It is to be noted that this change is considerably delicate, so that the sensitivity and precision of detection is unavoidably suppressed at a low value.
As will be understood from the above, the reinforcing steel searching instruments of the three conventional techniques never employ a new technique of magnetizing the reinforcing steel embedded in the concrete, before the detection operation of the reinforcing steel searching instrument.
The principle of operation of the above conventional techniques will be discussed. First, the sensor coil 61 is represented generally as shown in FIGS. 15 and 16. The self inductance L.sub.1 of the sensor coil 61 is given by Eq. 1. EQU L.sub.1 =.PHI..sub.1 /I.sub.1 N.sub.1 (Eq. 1)
where I.sub.1 is current to the coil; N.sub.1 is the number of turns of the coil; and .PHI..sub.1 is the magnetic flux.
The operational relationship between the coil 61 and the reinforcing steel 64 (not magnetized)is represented in FIG. 16, in which the magnetic flux 62 absorbed into the reinforcing steel 64 is indicated by .DELTA. (delta). As a result of this operation, the self inductance L .sub.1 ' of the coil 61 is changed and given by Eq. 2. EQU L.sub.1 '=(.PHI..sub.1 -.DELTA.)/I.sub.1 N.sub.1 (Eq. 2)
Thus, in the conventional techniques in which no magnetization of the reinforcing steel is carried out, the self inductance of the sensor coil is suppressed at a low value, and therefore the electromotive force at the sensor coil is considerably small thereby lowering the sensitivity and precision of detection of the embedded reinforcing steel in the concrete. Accordingly, the above-discussed or other conventional techniques for searching the reinforcing steel in the concrete are low in sensitivity and precision of the detection, and additionally such low in searchable depth of the reinforcing steel in the concrete as only 5 to 6 cm. This makes considerable troubles for the business world of constructors and builders and the authorities concerned.